She was born on March 18, 1999 in Seoul, South Korea.
Koeun said in a Ceci 2015 interview that she was a Track and Field athlete in elementary school. She was a registered 80-m track and field athlete in the Korea Associations of Athletic Federation, who represented Gyeonggi Bongilcheon Elementary School in Paju, Gyeonggi Province. She stopped competing now and is no longer re-registered, so her KAAF profile says she is a "retired player."
Koeun joined SM Entertainment in 2011, training for 7 years as of 2018. She was part of the variety show Mickey Mouse Club broadcasted through Disney Channel Korea in 2015. In 2016, she was part of the "Who's the best?" segment of My SMT along with fellow SM Rookies Hina and Ningning, and former SM Rookies Yiyang.
Later 2017, Koeun with fellow SM Rookies Hina and Ningning sang several OST for an MBC animation titled "Shining Star". They sang the opening theme "Shining Star", sang original songs titled "Dream Dream Dream", "Someday" and "Run and Run." They also covered Girls' Generation's "Into The New World", "Gee", f(x)'s "Ice Cream", BoA's "Moto", and SES' "I'm Your Girl." It is confirmed that one of the OST of Shining Star, Girls' Generation-TTS' "Checkmate" was covered and sung alone by Koeun. Koeun also sang with an unknown male vocalist for a duet version of "Someday."
"Think positively, act aggressively."
"The cute maknae Koeun is now Ko-leader."